Flatness measurement system for metal strip

ABSTRACT

The invention relates to a flatness measurement and control system for metal strip, which makes it possible to obtain improved strip or coil quality by a simple and effective measurement of departures from flatness and to control the finishing parameters through the evaluation of a line pattern on the strip surface or on the end face of a coil as it is coiled.

FIELD OF THE INVENTION

[0001] The invention relates to a flatness measurement and controlsystem for metal strip and for the end faces of the coil when coilingstrip.

BACKGROUND AND PRIOR ART

[0002] The contact measurement usually used in cold strip mills is onlypossible in the hot strip field at the expense of substantial outlay onmaintenance, because of the high strip temperature of from about 1000°C. Contact measurement on the end faces of a coil being formed in acoiler is also not possible. It is therefore difficult, if notcompletely impossible, to coil strip in such a way that in the coilevery turn lies exactly over the preceding one so that flat end facesare obtained. And even in cold strip mills efforts are made to avoidcontact measurement, since the mechanical measuring elements have only alimited life.

[0003] Strip flatness is therefore preferably measured without contact.For example, it is known to measure departures from flatness by means ofspots of light projected on to the strip. The position in space of thelight spot produced on the surface of the strip, preferably by means ofa laser beam, is detected using a range finder.

[0004] The two plane position coordinates of a particular point on thesurface are known from the position of the scanning or illuminating beamrelative to the surface of the strip. The height coordinate of the pointon the surface which is currently being measured is detected by aposition-sensitive sensor. The position of the image point on the sensorvaries simultaneously with the height coordinate.

[0005] Using a large number of sources of radiation and sensors aflatness image can be built up over the whole width of the strip whichis made up from the results of measurement of the spots of lightprojected on the strip at particular distances apart. Nevertheless, inthis method the regions between the points of light are not detected andin the case of continuous strip form strip-shaped measurement gaps inwhich the flatness is not determined. Moreover, this can result inmeasurement errors, for example through wobbling or flattering of thestrip being detected by the measuring method as unevenness of the strip.

[0006] In the automobile industry it is known to measure relativelysmall surfaces using the moiré technique. In this method an interferencepattern is produced oil the surface of the object by means of a lightsource. The interference pattern is detected using a CCD (charge-coupleddevice) camera. The camera is arranged so that an angle is formedbetween the light source, the surface and the camera. By the use of areference grid in the image plane a so-called moiré effect is obtainedby superposition of the detected pattern and the reference pattern. Theheight differences can be determined quantitatively from the moirélines.

[0007] The moiré technique provides more accurate measurement resultsthan measurement using spots of light, and moreover it coverssubstantially the whole of the surface to be measured and avoids themeasurement gaps mentioned above. However, its use involves problems,particularly in a hot strip mill.

[0008] To determine the height differences of the rolled strip acomplicated conversion of the pattern detected by the camera isnecessary. The height differences pictured as moiré lines cannot beconverted into quantitative measured values in real time.

[0009] But in a rolling mill train rapid results are precisely what isrequired from measurements, since otherwise it is hardly possible to usethe measurement for direct adjustment of the rolling parameters so as toimprove the flatness of the continuous strip. Moreover for industrialapplication the fine interference patterns are lacking in contrast andintensity.

[0010] In the case of conventional metal strip mill trains, in which thestrip flatness is measured by one of the above-mentioned methods,departures from flatness are not measured from the cooling line, andthis can result in considerable loss in quality.

OBJECT OF THE INVENTION

[0011] The object of the invention is to provide a system which allowsstrip quality to be improved by a simple and effective measurement ofthe strip flatness and permits fine control of the rolling and/orcoiling parameters.

SUMMARY OF THE INVENTION

[0012] To this end, a line pattern is produced on the measurementsurface and/or on the end face of a coil being formed, the line patternis detected by a camera which can resolve the line pattern, and themeasurement data obtained are compared with a reference measurement. Bymeans of a process control computer the measurement results are directlyconverted into control parameters for the finishing train and the coilerand coordinated.

[0013] By measurement surface is to be understood here the surface ofthe strip or, in the case of coiling, the end face of a coil made up ofa larger or smaller number of turns of the strip.

[0014] A projector produces by projection, for example through a slide,a line pattern on the strip surface or the end face of the coilcorresponding to the resolution of the camera. For this purpose theprojector is mounted above the metal strip and projects the line patternon the surface of the metal strip at an angle to the vertical, so thatthe lines preferably run transverse to the strip surface andconsequently take in the whole width of the strip.

[0015] A CCD camera having a resolution of, for example, eight pixelsper line detects the lines running transversely across the stripsurface. In the event of absolute flatness of the strip a uniformpattern of straight lines with constant line spacing is formed.

[0016] Deviations of the strip surface from the ideal plane bring abouta change in the spacing between the lines in the region of theunevenness. This change is recorded by the camera. It can be convertedby calculation in a simple manner into differences in height by acomparison with an ideal pattern.

[0017] In a similar manner to the measurement of flatness on thecontinuous strip, the flatness of the end faces during coiling can bemonitored and ensured by means of the measuring system of the invention.The end face of the coil being formed in the coiler then corresponds tothe strip surface.

[0018] The system in accordance with the invention makes possible arapid determination of the actual height differences of the stripsurface and in this way permits measurement of continuous lengths ofstrip in real time. This has the advantage that the measurement resultspermit the rolling parameters to be adapted immediately after theappearance of an unevenness.

[0019] The invention makes possible a measurement which is insensitiveto spurious measurement results. Such spurious results are obtained withconventional measuring systems for example as a result of movement ofthe whole strip surface relative to the height coordinate (flutterirg).Furthermore, the invention allows the transverse arching of the strip tobe determined. Conventional measuring systems only measure the length ofthe strip fibres. In addition the intensity and thickness of themeasurement lines can be adapted to meet different conditions. Theproblems of the fine, low-intensity and low-contrast moiré lines do notoccur.

[0020] The system of the invention is particularly suitable for making ameasurement on the strip emerging from the finishing stands combinedwith a measurement of the strip on the coiler. By this arrangementvariations in the flatness of the strip due to cooling of the stripbetween the finishing stands and the coiler can be detected andevaluated for flatness control.

[0021] The measurement data can be evaluated for control of thefinishing stands and of the coiler and for control of the coiling line.

[0022] Measurement results which incorporate a departure from anintended value bring about an immediate and interdependent adaptation ofthe parameters for the finishing stands, the cooling line and thecoiler.

[0023] Besides its use for measurement of flatness in a finishing trainthe system in accordance with the invention can also be used insubsequent production lines, for example in the control of stretchstraightening devices and in pickling lines.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be described in more detail, by way ofexample, with reference to an embodiment illustrated in the drawings, inwhich:

[0025]FIG. 1 shows the production and detection of the measurement lineson a length of strip;

[0026]FIG. 2 shows a projector and a camera arranged after a finishingtrain;

[0027]FIG. 3 shows the projector and the camera arranged before a coilerpit; and

[0028]FIG. 4 is a block diagram of the flatness control system.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0029] Measurement lines 2 running transverse to tie strip 1 areproduced on the measurement or strip surface 4 using a projector 3.

[0030] The measuring arrangement is disposed in ore case in the run-outfrom the finishing stands 6 and in the other case before the coiler 7,on an instrument case 13. The CCD camera 5 is located on the side of theinstrument case nearer to the coiler 7, in a water-cooled housing. Theprojector 3 is located on the side of the instrument case remote fromthe coiler 7. To remove heat the housing is cooled with air. The coolingof the projector 3 and of the camera 5 is necessary to remove theirintrinsic heat and the radiant heat from the strip 1, which is at about1000° C.

[0031] The camera 5 and the projector 3 are arranged in successionrelative to the direction of travel of the strip and are aimed at aregion of the strip located between them, on which the line pattern isproduced and sampled. The projector used may, for example, be a xenonlight source, which produces an easily readable line pattern even on ahot slab.

[0032] Unevennesses on the strip surface 4 cause the measurement lines 2to follow an irregular course or to depart from geometric straightness.

[0033] By means of a CCD camera 5 the measurement lines 2, andconsequently also the changes in their course caused by unevennesses,are detected. After it has been detected the measurement image iscompared by computer with a previously recorded reference pattern. Theheight differences and the parameters for the control of the finishingtrain are derived directly from the deviations.

[0034] A complete picture of the flatness of the strip 1 is therebyobtained as it moves along in the direction of the arrow.

[0035] From the diagram of the flatness control system (FIG. 4) thedesign in accordance with the invention can be seen. The hot strip 1passes through the finishing rolls 6 and the strip cooling line 8 to thecoiler 7 in the coiler pit. In the run-out from the finishing rolls 6the flatness of the hot strip is detected, analysed and evaluated forcontrol of the last stands of the finishing rolls (roll bending aidtilting). This internal flatness-control loop 9 is supplemented by anexternal flatness-control loop 10. By a measurement of the stripflatness after the strip cooling life 8 before the coiler 7 the externalflatness-control loop 10 is designed for adaptation of the intendedvalue of the internal control loop.

[0036] Using the measured values detected after the strip cooling line afirst secondary control loop 11 is also produced which permits theintended value for the cooling line 8 to be adapted and a secondsecondary control loop 12 which permits the intended value for thecoiler tension 7 to be adapted.

[0037] Altogether, the detection and control in accordance with theinvention can be used effectively to achieve a high strip

What is claimed is:
 1. Method for measuring the flatness of metal stripor of the end face of the coil when coiling strip, wherein a linepattern (2) is produced on the measurement surface (4) by means of aprojector and is detected directly by a camera (5)
 2. Method as claimedin claim 1 , wherein the line pattern (2) is compared by computer with areference pattern after its detection by the camera (5).
 3. Method asclaimed in claim 1 or claim 2 , wherein the measured values are used forcontrol of a finishing train.
 4. Rolling mill train comprising afinishing stand (6), a strip cooling line (8), a coiler (7) andmeasuring means located before and after the strip cooling line formeasuring the strip flatness, which is coupled with the means forcontrolling the finishing stand (6), the strip cooling line (8) and thecoiler (7).
 5. Rolling mill train as claimed in claim 4 , wherein theflatness of the strip is detected in the run-out from the finishing line(6) and evaluated for control of the last stands of the finishing rolls,and the measured values of the flatness measurement are used to modifythe flatness after the strip cooling stage (8) and before the coiler(7).
 6. Rolling mill train as claimed in any one of claims 1 to 5wherein, using the measured values detected after the strip cooling line(8), a first secondary control loop (11) is produced which permits theintended value for the cooling line (8) to be adapted.
 7. Rolling milltrain as claimed in any one of claims 1 to 6 wherein, using the measuredvalues detracted after the strip cooling stage (8), a second secondarycontrol loop (12) is produced which permits the intended value for thecoiler tension (7) to be adapted.